A taxonomic revision of Chloromonas reticulata (Volvocales, Chlorophyceae), the type species of the genus Chloromonas, based on multigene phylogeny and comparative light and electron microscopy

Chloromonas reticulata (Goroschankin) Gobi, the type species for Chloromonas, was taxonomically revised using nine strains previously assigned to the species, based on light and transmission electron microscopy and multigene phylogenetic analyses. Based on vegetative cell and papilla shape, eyespot form and position, and the ultrastructure of the eyespot and pyrenoid, the nine strains were classified into four species: C. reticulata; C rosae (H. & O. Ettl) H. Ettl; C. chlorococcoides (H. Ettl & K. Schwarz) Matsuzaki, Y. Hara & Nozaki comb. nov.; and C. typhlos (Gerloff) Matsuzaki, Y. Hara & Nozaki comb. nov. The taxonomic revision was supported by the robust monophyly of the species resolved in phylogenetic analyses using a combined dataset that included 18S ribosomal (r) RNA, ATP synthase beta-subunit, P700 chlorophyll a-apoprotein A1 and A2 genes, and nuclear rDNA internal transcribed spacer 2 (ITS2) sequences. The phylogenetic analyses demonstrated that the pyrenoid-lacking species C. reticulata and the pyrenoid-containing species C. chlorococcoides, which has a dense pyrenoid matrix associated with starch grains, formed a robust monophyletic group to which the pyrenoid-lacking species C. rosae and the pyrenoid-containing species C. typhlos were basal. Comparison of the C. reticulata and C. chlorococcoides nuclear rDNA ITS2 secondary structures revealed two compensatory base changes in the conserved helix III region, suggesting a separation between these sister species.